Niemann-Pick disease, type C (NPC) is a pan-ethnic, autosomal recessive lysosomal storage disease with a prevalence between 1/120,000 and 1/150,000. Because of the devastating effects of this illness on patients and families, the public health burden of this disease is disproportionate to its prevalence. Most patients experience a progressive dementia accompanied by vertical supranuclear gaze palsy ataxia, dysarthria, dysphagia, dystonia and in some cases, epilepsy and cataplexy. Several potential human therapeutic agents have been studied in vitro and in animal models of NPC. Before human therapeutic trials can be pursued, it is essential to define the clinical course of NPC, and to identify clinical, neuropsychological and laboratory markers of disease burden and progression. The purpose of this study is to test the hypothesis that patients with NPC will demonstrate a specific pattern of neurocognitive deficits that will be present prior to development of significant neurological deficits and that will correlate with disease progression. Subjects will be recruited from participants in studies at two network sites - the NIH Clinical Center and Mayo Clinic, and will be examined using a standardized set of instruments on an annual basis for the five years of the longitudinal study.